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d。 For example； our Oort Cloud seems to have been populated by gravitational ejections of icy worldlets from the vicinity of Uranus and Neptune。 If there are no planets that play the role of Uranus and Neptune in systems otherwise like our own； their Oort Clouds may be much more thinly populated。 Stars in open and globular stellar clusters； stars in double or multiple systems； stars closer to the center of the Galaxy； stars experiencing more frequent encounters with Giant Molecular Clouds in interstellar space； may all experience higher impact fluxes at their terrestrial planets。 The etary flux might be hundreds or thousands of times more at the Earth had the planet Jupiter never formed—according to a calculation by George Wetherill of the Carnegie Institution of Washington。 In systems without Jupiter…like planets； the gravitational shield against ets is down； and civilization…threatening impacts much more frequent。
To a certain extent； increased fluxes of interplanetary objects might increase the rate of evolution； as the mammals that flourished and diversified after the Cretaceous…Tertiary collision wiped out the dinosaurs。 But there must be a point of diminishing returns： Clearly； some flux is too high for the continuance of any civilization。
One consequence of this train of argument is that； even if civilizations monly arise on planets throughout the Galaxy； few of them will be both long…lived and non…technological。 Since hazards from asteroids and ets must apply to inhabited planets all over the Galaxy； if there are such； intelligent beings everywhere will have to unify their home worlds politically； leave their planets； and move small nearby worlds around。 Their eventual choice； as ours； is spaceflight or extinction。
CHAPTER 19 REMAKING THE PLANETS
Who could deny that man could somehow also make the heavens；
could he only obtain the instruments and the heavenly material？
—MARSILIO FICINO； 〃THE SOUL OF MAN〃 （CA。 1474）
In the midst of the Second World War； a young American writer named Jack Williamson envisioned a populated Solar System。 In the twenty…second century； he imagined； Venus would be settled by China；* Japan； and Indonesia； Mars by Germany； and the moons of Jupiter by Russia。 Those who spoke English； the language in which Williamson was writing； were confined to the asteroids…and of course the Earth。
* In the real world； Chinese space officials are proposing to send a two…person astronaut capsule into orbit by the turn of the century。 It would be propelled by a modified Long March 2E rocket and be launched from the Gobi Desert。 If the Chinese economy exhibits even moderate continuing growth—much less the exponential growth that marked it in the early to mid…1990s—China may be one of the world's leading space powers by the middle of the twenty…first century。 Or earlier。
The story； published in Astounding Science Fiction in July 1942； was called 〃Collision Orbit〃 and written under the pseudonym Will Stewart。 Its plot hinged on the imminent collision of an uninhabited asteroid with a colonized one； and the search for a means of altering the trajectories of small worlds。 Although no one on Earth was endangered； this may have been the first appearance； apart from newspaper ic strips； of asteroid collisions as a threat to humans。 （ets impacting the Earth had been a staple peril。）
The environments of Mars and Venus were poorly understood in the early 1940s； it was conceivable that humans could live there without elaborate life…support systems。 But the asteroids were another matter。 It was well known； even then； that asteroids were small； dry； airless worlds。 If they were to be inhabited； especially by large numbers of people； these little worlds would somehow have to be fixed。
In 〃Collision Orbit；〃 Williamson portrays a group of 〃spatial engineers；〃 able to render such barren outposts clement。 Coining a word； Williamson called the process of metamorphosis into an Earth…like world 〃terraforming。〃 He knew that the low gravity on an asteroid means that any atmosphere generated or transported there would quickly escape to space。 So his key terraforming technology was 〃paragravity；〃 an artificial gravity that would hold a dense atmosphere。
As nearly as we can tell today； paragravity is a physical impossibility。 But we can imagine domed； transparent habitats on the surfaces of asteroids； as suggested by Konstantin Tsiolkovsky； or munities established in the insides of asteroids； as outlined in the 1920s by the British scientist J。 D。 Bernal。 Because asteroids are small and their gravities low； even massive subsurface construction might be paratively easy。 If a tunnel were dug clean through； you could jump in at one end and emerge some 45 minutes later at the other； oscillating up and down along the toll diameter of this world indefinitely。 Inside the right kind of asteroid； a carbonaceous one； you can find materials for manufacturing stone； metal； and plastic construction and plentiful water—all you might need to build a subsurface closed ecological system； an underground garden。 Implementation would require a significant step beyond what we have today； but—unlike 〃paragravity〃—nothing in such a scheme seems impossible。 All the elements can be found in contemporary technology。 If there were sufficient reason； a fair number of us could be living on （or in asteroids by the twenty…second century。
They would of course need a source of power； not just to sustain themselves； but； as Bernal suggested； to move their asteroidal homes around。 （It does not seem so big a step from explosive alteration of asteroid orbits to a more gentle means of propulsion a century or two later。） If an oxygen atmosphere were generated from chemically bound water； then organics could be burned to generate power； just as fossil fuels are burned on the Earth today。 Solar power could be considered； although for the main…belt asteroids the intensity of sunlight is only about 10 percent what it is on Earth。 Still； we could imagine vast fields of solar panels covering the surfaces of inhabited asteroids and converting sunlight into electricity。 Photovoltaic technology is routinely used in Earth…orbiting spacecraft； and is in increasing use on the surface of the Earth today。 But while that might be enough to warm and light the homes of these descendants； it does not seem adequate to change asteroid orbits。
For that； Williamson proposed using anti…matter。 Antimatter is just like ordinary matter； with one significant difference。 Consider hydrogen： An ordinary hydrogen atom consists of a positively charged proton on the inside and a negatively charged electron on the outside。 An atom of anti…hydrogen consists of a negatively charged proton on the inside and a positively charged electron （also called a positron） on the outside。 The protons； whatever the sign of their charges； have the same mass； and the electrons； whatever the sign of their charges； have the same mass。 Particles with opposite charges attract。 A hydrogen atom and an anti…hydrogen atom are both stable； because in both cases the positive and negative electrical charges precisely balance。
Anti…matter is not some hypothetical construct from the perfervid musings of science fiction writers or theoretical physicists。 Anti…matter exists。 Physicists make it in nuclear accelerators； it can be found in high…energy cosmic rays。 So why don't we hear more about it？ Why has no one held up a lump of antimatter for our inspection？ Because matter and anti…matter； when brought into contact； violently annihilate each other； disappearing in an intense burst of gamma rays。 We cannot tell whether something is made of matter or anti…matter just by looking at it。 The spectroscopic properties of； for example； hydrogen and anti…hydrogen are identical。
Albert Einstein's answer to the question of why we see only matter and not anti…matter was； 〃Matter won〃—by which he meant that in our sector of the Universe at least； after almost all the matter and anti…matter interacted and annihilated each other long ago； there was some of what we call ordinary matter left over。* As far as we can tell today； from gamma ray astronomy and other means； the Universe is made almost entirely of matter。 The reason for this engages the deepest cosmological issues； which need not detain us here。 But if there was only a one…particle…in…a…billion difference in the preponderance of matter over anti…matter at the beginning； even this would be enough to explain the Universe we see today。
* If it had been the other way； then we and everything else in this part of the Universe would be made of anti…matter。 We would； of course； call it matter—and the idea of worlds and life made of that other kind of material。 the stuff with the electrical charges reversed； we'd consider wildly speculative。
Williamson imagined that humans in the twenty…second century would move asteroids around by the controlled mutual annihilation of matter and anti…matter。 All the resulting gamma rays； if collimated； would make a potent rocket exhaust。 The anti…matter would be available in the main asteroid belt （between the orbits of Mars and Jupiter）； because this was his explanation for the existence of the asteroi